This invention is in the field of determination of subsurface earth structure resulting from the application of pressurized fluid at one or more points in the earth, by the evaluation of pressure waves in a well connected to the point of application and by the evaluation of seismic earth movement. More particularly, it is concerned with determining the geometry and orientation of fractures in the earth.
Hydraulic fractures are produced in oil and gas fields, in solution mining operations, in fresh water aquifers, and certain other resource recovery operations for the purpose of extracting more fluid from the earth than is possible in wells that not have been hydraulically fractured. Hydraulic fractures are also used to more effectively disperse liquid waste into subsurface formations when these liquids are pumped into disposal wells. Generally stated, hydraulic fractures increase the hydraulic conductivity of the subsurface geologic formation, permitting greater amounts of fluid to be injected and extracted than would be the case if the fractures were not present.
Experience has shown, and it is now commonly accepted, that most hydraulic fractures are large, planar structures with surface areas from tens to many thousands of square meters. Because of their economic importance in resource recovery and waste disposal, it is often desirable to establish the orientation and, if possible, the dimensions of the fracture plane in the earth. "Geometry" and "dimensions" as used in this invention refer to the length, height and width of the fracture, where length and height are the two major axes in the plane of the fracture and width is measured perpendicular to the fracture plane. Knowledge of fracture orientation and dimensions permits wells to be drilled in optimal locations to take advantage of the non-uniform drainage or injection patterns than hydraulic fractures produce. In this way it may be possible to extract more of the resources in a field using a smaller number of wells that would be possible if fracture geometry were not known. Fracture orientation can also be used to determine the orientations of the principal stress directions in the earth around the fracture. Knowledge of stress directions in the earth is important for certain engineering purposes, such as tunneling and blasting, and in the study of regional geologic structures and earthquakes. Furthermore, information about the rate and directions of hydraulic fracture growth can be used in improving the design and production of the fractures, thereby resulting in economic savings to the individuals and organizations who use hydraulic fractures in their operations.